docs: def createReference(self, fromnode, tonode, edge_data=None):
          """
          Create a reference from fromnode to tonode
          """
          if fromnode is None:
              fromnode = self
          fromident, toident = self.getIdent(fromnode), self.getIdent(tonode)
          if fromident is None or toident is None:
              return
          self.msg(4, "createReference", fromnode, tonode, edge_data)
          self.graph.add_edge(fromident, toident, edge_data=edge_data)